The present invention relates generally to an apparatus and method for tightening fasteners with torque limiting.
In the course of many mechanical assembly operations, it is important to tighten a fastener with a specified torque, so that the optimal potential of the fastener is utilized and/or overload of parts fastened together is prevented in order to avoid danger of damage to either the parts or the fastener itself. One example of such operation, where correct tightening of the fasteners is of great importance, is the mounting of optical scopes into scope rings. It is common knowledge that the scope ring fasteners should be tightened to strictly controlled values. Exceeding these values may cause damage to the thin wall scope bodies, while a smaller than required tightening torque applied to the fasteners may result in insufficient clamping force, which, in turn, may cause movement of the scope under force of firearm recoil, with the loss of the preset scope setting as the direct result of such scope movement relative to the rings.
It has been known to use special torque-limiting or torque setting devices, such as adjustable or pre-set torque drivers of different configurations. While adjustable torque drivers allow the user to set up such devices to desired tightening torque values using a mechanically based scale normally incorporated into such device, these devices suffer from certain inconsistency and relatively low repeatability of the same settings, as the human eye is often not capable of precisely identifying the same spot on the relatively low resolution scale typical for such mechanical devices. In addition, the vast majority of such devices are based on the use of “cam-over” and “slip clutch” systems and precision springs, which must be manufactured with a high degree of accuracy to assist in maintaining precision and consistency of the torque driver. As a consequence of having precision parts, such devices may be expensive and may also require labor intensive calibration. An example of such device is shown in U.S. Pat. No. 7,383,756. While an adjustable torque driver can be made with a digital readout, which improves resolution of the scale, and may utilize a load cell as a torque registering element instead of the mechanically based click-clutch system, the cost of such a digital adjustable torque driver is relatively high, which limits its use.
Pre-set torque drivers, for example the one shown in U.S. Pat. No. 6,662,693 typically have much simpler construction and lower cost, but are only able to deliver a single fixed torque setting, although, despite more simple construction, with a higher degree of precision and consistency in comparison with the adjustable torque driver. This driver design is typically not intended to be re-adjusted by the user, and therefore, can only deliver a single factory pre-set torque limiting value.
The single setting limitation is also considered to be a certain weakness by the users, who may often need to use more than one pre-set torque driver to cover the need for several fastening applications requiring at least several fixed torque settings, in which case the cost to the user to own these tools becomes considerable.
Another known pre-set “break-over” torque driver design, for example the one shown in Mountz, Inc. Catalog section 3.6-3.9, or described by U.S. Pat. No. 6,138,539, possesses high accuracy potential and capability to be used in difficult to reach places, along with availability of certain adjustments by the user, which can be realized thru adjustment of the internal compression type spring. However, in order to guarantee the accuracy of adjustment, this type of pre-set torque driver, which has no adjustment scale, must be set up with the additional use of a torque analyzer. In addition, even with this capability to be set up to the new torque limiting value, this design can still deliver only one torque setting between adjustments, and the need to use a torque analyzer makes practical implementation of this driver very expensive for the user. While the manufacturer offers a selection of pre-set driver handles, which can be switched to obtain different torque limiting settings without the use of a torque analyzer, the cost of the base “break-over” type torque driver combined with the need to have several optional handles remains quite high for the user.
Therefore, it would be beneficial to propose a torque tightening method and also, an improved configuration of the torque driver capable of delivering more than one consistent and highly repeatable fixed torque setting for tightening the fasteners. It would be also very beneficial if such torque driver design would allow certain torque setting adjustment capability, but in combination with a simple, easy to manufacturer and low cost design.